Single-sideband system



Feb.4 17, 1959 F, DE JAGER 2,874,222

SINGLE-SIDEBAND SYSTEM Filed oct. 24, 1955 2 sheets-sheet 2 Anrufung FIL-rin. LMTEQ smoke-swawmo moouLA-roe;

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MaouLA-ralz E MaDuLA'raRfy @LTER 65 am el es y 75 7s w23-MQ AGENT United StateN Patent 2,374,222 SINGLE-SIDEBAND SYSTEM FrankDe Jager, Eindhoven, Netherlands, assigner, ny

mesnerassignments, `to N orth American Philips Coinpany,"lnc.,` New York, N. Y., a corporation ofv Dela- Ware` Application October 25, 1955, Serial No. .542,222`

Claims priority, 'application N etherlands October 25,-'1954 15 Claims. (Cl. 179-155) The invention relates to a single-sideband system comprisinga transmitter and a receiver for the transmission of speech signals, `in, which thetransmitter and the 'receivervare provided with avband compressor and arband l Such single-sideband systems which achieves onthe one hand an appreciable Jeconomyofthefrequencyband and onthe otherihand provides good .transmission quality, more particularly withl respect to` the intelligibility` and lthe identication of fthe :.voices.

In, :accordancewithf xthe t invention the i single-sideband t signal is fed on the one hand to an envelope detectorin order Ato Vobtain i a signal `envelope andion; thelotherk hand,

through, a band compressor, to,` an. amplitude l modulator controlled bytthersignal' envelopefobtainedfrom the enfV velopedetector; theoutput signal `of the amplitude modnv lator tbeingtransmittedtothe receiver,Y which; in order l tonre-obtainthe initial single-sidebandsignal, is provided with a bandexpander. i

Int order f tov further.: improve Y thev transmissionV quality the speech .signal is converted'into single-sideband fsi'gnals representingdiiferent formant ranges, atleast one=of `the lower formantranges rbeingftransmitted `by means ofthe measure according to the invention;v t

The invention and its advantages` will now be described more `fullylwith Yreference to the figures.'

Figs..` 1 "and 2 show one embodiment of 't1-transmitterc andfa receiver respectively for' use in al sing-le-sideband system` according tothel'invention.`

Figsr. 3 and 4 -show'a variant of Vthefsingle-sideband transmitter and-r receiver:` shown inl Figs. 1 and "2.

Figs. 5 and :6.show,one: embodimentfof a'ftransmitterV andafreceiverV respectively, in :which the speech signal is converted .into .single-'sidebandzsignals representing; dfferent'; formantfranges iand= The transmitter andfthereceiver `shownfinFigsJ and 8 represent: improvements of the transmitter-and the receiverrshown;inFigs.` Sand 6; .Y

'Ihetransmitter andltheareceiver shoWnLin'FgS. 1 and 2 .form part "ofi a1'carrienwaveltelephone.system suitable fonthetransmissionlrof sideband modulation:

Inthetransmitter shown in' Figli 1 'the-signals obtained from arrmicrophone l'aresupplied through a filter-2 passingfthelspeechA signals` off`300-to 3200 CJS., subsequently tolamplification in -anamplier 3, to la single-sideband modulator 14,' having an=`oscillator=5` of vfor `example 60 kc./s., connected'thereto, and a'sin'gle-*sideband "ilter"6; which@ passes for example nthe/upper sdeband (60.3 to 63;`2`*kc./s.).= The sngle-sideband signal lthus'ob'tained isfedil'toa b'andfc'ompressor 17,7 which is connectedebyway of a filter 8 and a group modulator9with"anoscilltor 10afnd anfoutpnt iilterill 5to A:a transmission cablelzf' speech' signals Vby means0 ot`single-` 2; In accordance with` the invention.the-single-sideband signal in the bandcompressor 7 issupplied'on the'one hand, in order` to obtainfa signal envelope, to an envelopey detector'13, and one Vthe other `hand`by way of a band.

compressor 15'to an amplitude modulator 116, which is governed by the signal Yenvelope obtained fromV the-envelope detector 13. favourable to provide a-'limiter 14 before the band compressor.

The band compressorlS iis-constructed inthe form of a frequencyedivision circuityasf'a countin'gcircuit arrange- Y ment Ycomprising a counting tube,1provide'd with a' ysuitable output filter' to suppressfunwanted distortiony prod! ucts. Ifthe division factor is'for example 10, a singlesideband signal is obtaiedffromthesingle-sideband sigrespect to the bandwidthof Vthefinitial single-sidb'aud signal (2.9 kc./s.) -by afactorlO.

In the amplitudemodulator v16"th`e single-sideband signal'from 6.03 to 6.32 l `c./s.` of vapproximately constant amplitude is modulated in Vamplitude by'thefoutput signal of the envelope detector113, which isl constitutediby an amplitude detector 17 and a next-following low-pass filter y18 having a cut-ofrequencyof vfor'example-'SOO''c./s.

andSsupplied through the groupmodultor 9to thejtransmission cable 12; y

Cempared withltheginitial 'single-stamm Signat a? ma. terial economy in bandwidth is thus obtained, whilstifthe speechn signal thus transmittedby "bandfcornpressionf can beT reproduced'vat'theireceiver end-within fairly` good qualityvIn order to improve thetransmissionfquality"With-ref spect to thepropertyof-the limiter `14 yto pass those-speeclr` components of the components supplied thereto which have. a-larger .amplitude better than the other components,

it has been found to be favourable-to"supply*thespeechi4 signals fromthe microphoneflby Iway of apre-emphasis network? 19 to thesingle-sidebandr modulator 4, sincetlh'e pre-emphasis network 19 providesfanA 'amplitude'fequalilzr' tionzof the transmitted signals;i `so "thatV in the limited single-sideband 4signalffalso ,the higher speech"- frequency components ofthefspeech frequencyl band of foreram`` ple -2000 to 3200 C./S.arefairlyfwell represented.

Instead of using -the frequency-division arrangement shown in the embodimentfuse-may be-madeof other fre:

quency ydivision arrangements.` Th'us, use may, for ex# I ample, bemadeof'a counting circuit comprising a 'plurality of pulse generators exciting vont=`,'--anotlier in succession,. each` of these pulse generators producing a Vdivision, by 'a` factor 2 (sofcalled binary counting circuit). Forthefre# quency` division use may; furthermore vbe made advant'a# geously of an oscillator tu'ned approximately; to the desired subharmonic of the carrierhfrequency' of the"`single`v sideband 'signal Vand synchronizedby the single-sideband signal (cf. for example Proceedings of the l. R. Blof December 1944,pages 73010 737);

Fig.j 2 showsi the receiver 'co-operating "with:v the trans-` mitter shown in Fig. 1.

The incoming signals `from a transmission 'cable 12' are supplied subsequent to high-frequencyamplification inan amplifier'20, by way of a group demodulator'ZL com-- prising-Aan oscillator 22,`connectedrthereto, and an'output'v" filter 223, to* aband expander 24,`- comprising an outputfilter' 25 Tand convertingthe `single1sidebandi signal trans-V mitted `by bandcompressione'into the initialV frequency band Thesingle=side'b`and;signallthusfobtained lis* demodulated in a single-sideband"demodulator 26,vcomf prising Aalocal osci1la'tor27 'and alte'r 28`f`passing only,t the"speechfrcquency"band from'0f3 to 3.2`k`c.`/s'. and sup- In practice it Vhas been found 'toi be:

plied through a defemphasis network 29 and a low-frequency amplifier 30 to a reproducing device 31.

In order to re-obtain the initial single-sideband signal the incoming single-sidebandsignal is supplied in the expander 24 on the one hand to an envelope detector 32, which is constituted byan amplitude detector 33 and a low-pass lter 34, having a cut-off frequency of for example 300 C. /S., and on vthe other hand via a limiter 35 and a band expander 36 to an amplitude modulator 37, which is governed by the signal envelope obtained from the envelope detector 32. u

The band expander 36 is constituted by a frequency multiplication circuit, comprising a suitable output lter to suppress unwanted distortion products and to this end use may, for example, be made of frequency multiplication circuits of the kind used with frequency-modulation transmission (cf. for example French patent specication No. 972,594). If, in the embodiment shown, for example the frequency band of the single-sideband vsignal obtained from the limiter 35 is 6.03 to 6.32 kc./s., a single-sideband signal is obtained in the initial frequency band from 60.3 to 63.2 kc./s., subsequent to frequency multiplication by a factor'lO in a frequency multiplier 36 subsequent to amplitude modulation with the signal envelope in the amplitude modulator 36 this signal yields approximately the initial single-sideband signal., The intelligibility of the transmitted conversation is, in this case, satisfactory.

In the system shown transit time differences between the envelope channel and the band-compression or the band-expansion channel may produce a reduction of the transmission quality. If these transit time differences are a source of interference, one of these channels may be provided with a network equalzing the transit time differences.

A particularly simple band expansion arrangement is obtained by using a class C amplifying stage, which is controlled by the frequency-compressed single-sideband signal in this case, a suitable output filter permitting the attainment of approximately the initial single-sideband signal.

Figs. 3 and 4 show a variant of the single-sideband mitter shown in Fig. l in the construction of the' band' compressor 7. In this system the compression is obtained by supplying the single-sideband signal taken from the limiter 14 to the frequency discriminator 38, which is coupled with a frequency modulator 40, connected to an oscillator 39, for example a reactance tube. This bandv compressor 38,39, 40 is constructed in a manner such that a frequency variation of the limited single-sideband signal by way of the frequency discriminator and the frequency modulator 40 produces a correspondingly smaller variation of the oscillator frequency. In this case, for example, a frequency .variation of the'limited single-sideband was reduced to a ten times smaller variation of the oscillator frequency i. e. the band compressor factor was 10.

Fig. 4 shows the corresponding single-sideband receiver, comprisinga-band expander constituted by a frequency discriminator 41 and a subsequent frequency modulator 42, which -is coupled with a local oscillator 43, this band expander 41-43 thus corresponding in construction to the band compressor 38-40 used in the transmitter. However, this band expander 41-43 is proportioned to be such that a frequency variation of the limited singlesideband Asignal produces a. correspondingly larger variation of the oscillator frequency.- In the example shown, the band expansion factor is chosen to be 10 in order to rcobtain the initial single-sideband signal.

In the arrangement shown small differences between` the band compression factor and the band expansionfactor appear to be permissible, which is of particular importance for a simple construction of the band compressor and the band expander.

After the foregoing it will be obvious that the transmitter shown in Fig. l may be used in conjunction with a receiver shown in Fig. 4 or else the transmitter shown lin Fig. 3 may b e combined with the receiver shown in Fig. 2.

A further improvement in transmission quality may beA obtained by converting the speech signal into single-side# band signals representing different formant ranges, at least one of the lower formant ranges being transmitted rby means of the single-sideband system described above.

Such a single-sideband system is shown in Figs. 5 and 6: Fig. 5 shows the transmitter and Fig.v 6 shows the receiver.

In the transmitter shown in Fig. 5 the 'low-frequency amplier is connected to a band compressor 44, comprising three parallel-connected channels 45, 46, 47 with input lters 48, 49, 50, passing respectively formant ranges from 300 to 800 C./S., 800 to 2000 C./S. and 2000 to 3200 C./S. Each of these parallel-connected channels comprises the cascade combination of a single-sideband modulator 51, 52, 53, a single-sideband filter 54, 55, 56, a limiter 57, 58, 59, a frequency division stage 60, 61, 62 and an output filter 63, 64, 65, vwhich are connected in parallelwith the group modulator 9. The channel 45, associated with the lowest formant range from 300 to 800 C./S., comprises furthermore a modulator stage 66, connected to the frequency division stage 60'c'ontrolled by the output voltage of the envelope detector, constituted by an amplitude detector 67 and a subsequent low-pass lter 68, having a cut-off frequency of for example 300 C./S.

To the modulators 51, 52, 53vare connected oscillators 69, 70 respectively, having a frequencyof for example 57 kc./s. and 60 kc./s. to obtain the amplitude modulation of the speech frequencies lying in the various formant ranges: the single-side'hand filters 54, 55, 56 allow the upper sidebands in frequency bands from 57.3 to 57.8 kc./s., 60.8 to 62 kc./s. and 62 to 63.2 kc./s. respectively to pass. These single-sideband signals are divided, subsequent to limitation, in the frequency division devices 60, 61, 62, for example by a factor 10 and, subsequent to division, they produce single-sideband signals in the frequency bands from 5.73 to 5.78 kc./s., 6.08 to 6.2 kc./s. and 6.2 to 6.32 kc./s'. respectively. The frequency space introduced between the single-sideband signal from 5.73 to 5.78 kc./s. and the single-sideband signal from 6.08 to 6.2 kc./s. (i. e`. 0.3 kc./s.) is used for the transmission of the amplitude modulation of the single-sideband signal from 5.73 to 5.78 kc./s. In the example shown the output filter 63 of the'channel 45 may, to this end, have a minimum pass-band from 5.73 to 6.08 kc./s. Y

The output voltages of the channels 475, 46, 47 (only the signal envelope of the channel 45 is transmitted) are transmitted via the group modulator 9.and the output cable 12 to the receiver sho'wnl in Fig. 6.

In the receiver shown in Fig. 6, the output voltage of the group demodulator 21, in order to re-obtain the initial single-sideband signal, is supplied to a band-expander 71, comprising three parallel-connected channels 72, 73,`

74, which correspond to the band compressor channels 45, 46, 47 respectively in the transmitter.

Each of these channels72, 73, 74 comprises the cassequent low-pass filter 92,Y having a cut-olf frequency of for example 300C./S.

' Ioythe frequency Vtransformer stages 75, 76, 77 are connected `'oscilflato'is 93 "andj`94""1`f"esfpec`tivl producing frequencies .such -tl1at `l thefsirigle's'ide'band'-*signal of Athe lowest formant range with respect to fthe ysingle-sitiebaiid signals representing the V'further formant ranges-is shifted into-its initial .position. In fthe'exatnpleshownthe singlesideband signal fromf6.03 to 6.08`l c./ s., 1which'amplitilde-modulated by the signal envelope, isp'roduced at the output offthe ffilter 7S, having apass-'band of for example 6.03 to 6.38 kc./s., whilstthe filters "79-and 180 pass the single-sideba-ndsignals of constant amplitude lying'infthe adjacentffrequency bands from "6.108 to 6.2 kc./s. and "6.2 to6g32 Skcs/s.V These fsingle-s'idebandsignals yield, subsequent tofrequency multiplication in the frequency multipliers 84, 85, 186, =by afactor l0, the single-sideband signals in'Y the `initial lformantranges from 60.3 to 60.8 kc./s., 60.8 to 62kc./s. and 62 't'o163.2 kc./s. Y

The single-sideband signalV from 960.3 to 60.'8'kc./s., subsequent to amplitude modulation withthe signalenvelopefin .the amplitude modulator 90Yis supplied, together with the single-'sideband-"signals--Ifrom v'60.'8 to 62 kc./s. and 62 to 63.2 kc./s. to the single-sideband demodulator `26, from the output `circuit .ofwhich the transmitted speech signal is obtained.

In order to avoid interference signals during the speech intervalsgin :the .transmission channels for the higher formant ranges, it may, be. .advantageous to provide :a threshold device in these channels.

In this arrangement the speech signal is characterized by the three components in the various formant ranges, which determine together the speech signal with a satisfactory accuracy. By using the system described above an improvement in the transmission quality is obtained.

The dynamic power of the speech components, transmitted with constant amplitude, in the formant ranges from 800 to 2000 C./S. and 2000 to 3200 C./S. may be introduced at the receiver end in a simple manner by including an amplitude modulator 95 and 96 in each of the channels between the frequency multiplier 85 and 36 respectively and the output'lter 88 and 89 respectively, this modulator being controlled by the output voltage of the envelope detector 91 and 92 respectively ofthe channel 72. The dynamic power of the speech components in the formant ranges from 800 to 2000 C./S. and 2000 to 3200 C./S. appears to follow that of the lowest formant range.

If desired, instead of using the envelope of the speech component of the formant range from 300 to 800 C./S., use may be made of that of the formant range from 800 to 2000 C./S.

A further improvement in the transmission quality may be obtained by using the system shown in Figs. 7 and 8. This improvement consists in that the speech components of the various formant ranges are united in the correct intensity ratio.

In the transmitter shown in Fig. 7 the output voltages of the frequency multipliers 61, 62 in the band compression channels 46, 47 associated with the higher formant ranges are supplied to amplitude modulators 97, 98 which are controlled by the output voltage of an amplitude detector 99 and 100 respectively, connected to the output circuit of the single-sideband filter 55 and 56 respectively and of a subsequent low-pass filter 101 and 102 respectively, having a low cut-off frequency of for example 40 C./S. The level of the output signals of the channels 46, 47 is then determined by the level of the speech components supplied to the channels 46, 47.

In order to reproduce the speech components of the various formant ranges in the correct intensity ratio at the receiver end (cf. Fig. 8), each of the input filters 79, 80 of the channels 73 and 74 respectively is connected to an envelope detector, constituted by an amplitude detector 103 and 104 respectively and a low-pass filter 105 and 160 respectively, having a cut-off frequency of for @tempie 4o `'c1/Sgm, output fveiage" of which f comme a-"go'verning tube 107 and 108 respectively, connected' after the amplitude modulator 95 and 96 respectively in or'der rto control the level.

In order to avoid a reaction of the level of the channel 72 `on the signal levels of the channels 72 vand 74, the-signals Vfrom the envelope'detectors 91 and 92 in the ehannel 7-2 are supplied tothe amplitude modulators 95 'and 196 respectively via an automatic volume-control amplifier 109, the automatic volume control-voltage being obtained, for example by rectification of the signals supplied to the channel 72.

v T-Forlevel control lthegoverning tube 107 or 108 may, for example, vbeconnected between the amplifier 109 and the amplitude modulator 95 or 96 respectively.

Thefdyna'mic power control `and the level control in theA channels 73 and 74 do not substantially influence one anothersince they are operative in different frequency ranges.

By-using this single-sideband system a very satisfactory speech quality :is obtained.

Inorder-to improve the economy in bandwidth, `in the single-sideband system the band Ycompression factor may b'eraised and/or the frequency band of the envelope signal may vbe reduced: for example band compression factors from 40 to S0 maybe used and an economy of the band of the envelope to 100 or even `40 `C./ S. is possible.

What is claimed is:

l. A single-sideband transmission system comprising a transmitter having a source of a modulated single-sideband signal, an envelope detector circuit connected to receive said signal and produce therefrom an envelope thereof, a band compressor circuit connected to receive said signal and produce therefrom a frequency-compressed signal, and an amplitude modulator connected to modulate said frequency-compressed signal with said en- Yvelope thereby producing an output signal, and a receiver for receiving said output signal, said receiver comprising an envelope detector circuit connected to receive said output signal and produce therefrom an envelope thereof, a band expander circuit connected to receive said output signal and produce therefrom a frequency-expanded signal, and an amplitude modulator connected to modulate said frequency-expanded signal with the lastl named envelope.

2. A transmission system as claimed in claim l, in which said source of a modulated single-sideband signal comprises a speech signal source, a single-sideband modulator, and a pre-emphasis network connected between said speech signalsource and said single-sideband modulator, and in which said receiver comprises a single-sideband demodulator connected to the output of the lastnamed amplitude modulator thereby producing a demodulated signal, and a de-emphasis network connected to receive said demodulated signal.

3. A transmission system as claimed in claim 1, in

v which said source of a modulated single-sideband signal comprises an audio signal source, means for converting said audio signal into a plurality of signals having different frequency formant ranges, and a single-sideband modulator connected to produce said modulated single-sideband signal from at least one of said plurality of signals.

4. A transmission system as claimed in claim 3, including means for transmitting the remaining ones of said plurality of signals, and in which said receiver includes additional channels for receiving said remaining signals, each of said additional channels including a band expander circuit and an amplitude modulator connected to the output of the respective band expander circuit, and means connecting the output of said receiver envelope detector circuit as a modulating voltage to each of said receiver amplitude modulators.

5. A transmission system as claimed in claim 4, including a plurality of automatic volume-control ampli- 7 tiers connected respectively between said receiver amplitude modulators and said receiver envelope detector circuit.

6. A single-sideband transmitter,l comprising a source of a modulated single-sideband signal, Ian envelope detector circuit connected to receive said signal and produce therefrom an envelope thereof, a band compressor circuit connected to receive said signal and produce therefrom a frequency-compressed signal, and an amplitude modulator connected to modulate said frequency-compressed signal with said envelope.

7. A transmitter as claimed in claim 6, including an amplitude limiter connected to limit the amplitude of said signal as received lby said band compressor circuit.

8. A transmitter as claimed in claim 6, in which said band compressor circuit comprises a frequency division circuit.

9. A transmitter asclaimed in claim 6, in which said band compressor circuit comprises an oscillator tuned approximately to a subharmonic of the carrier wave of said single-sideband signal, and means connected toy synchronize said oscillator by said single-sideband signal.

10. A transmitter as claimed in claim 6, in which said band compressor comprises a local oscillator, a frequency modulator connected to frequency-modulate said local oscillator, and a frequency diseriminator connected .to control said frequency modulator.

11. A single-sideband receiver for receiving afre quency-compressed single-sideband signal, comprising an envelope detector circuit connected to receive said signal and produce therefrom an envelope thereof, a band expander circuit connected to receive-said signal and produce therefrom a frequency-expanded signal, and an amplitude modulator connected to modulate said frequency-expanded signal with said envelope.

12. A receiver as claimed in claim l1, including an amplitude limiter connected to limit the amplitude of said signal as received yby said band expander circuit.

13. A receiver as claimed in claim l1, in which said band expander circuit comprises a class C amplifier.

14. A receiver as claimed in claim 11, in which said band expander circuit comprises a frequency multiplication circuit.

15. A receiver as claimed in claim 11, in which said band expander circuit comprises a local oscillator, a frequency modulator connected to frequency-modulate said local oscillator, and ay frequency discriminator connected to control said frequency modulator.

Shore Nov. 5, 1940 Tunick Sept. 3, 1946 

